The present invention relates to a radiocommunication base station, in particular a station for a cellular network, more particularly a station for small cells (microcells or picocells) in an indoor environment (a station for installation indoors).
The antenna systems of radiocommunication base stations are normally required to radiate a wave that is polarised vertically relative to the ground, i.e. with a vertical electrical field vector. The reason for this is that the dipoles of mobile station antennas are usually oriented around the vertical when the stations are communicating. The vertical polarisation of the wave produced by the base station therefore optimises the power picked up.
As a general rule, it is desirable to integrate the antenna system with the structure of the casing of the station to limit installation costs associated with the use of connectors, cables and remote antennas.
For indoor applications, two configurations of the base stations (or their antenna systems if these are separate from the units which handle digital processing and the interface with fixed networks) are routinely used by installers: a wall-mounted configuration and a ceiling-mounted configuration.
The antenna is often a dipole (or a monopole) which radiates a wave whose electric field is polarised parallel to the axis of the dipole. Also encountered are microstrip printed circuit antennas in which the radiation pattern is more directional. For the electric field produced to be polarised vertically in the usual two configurations (wall-mounted and ceiling-mounted), the number of antenna systems has to be doubled, which is not economical and can give rise to problems of overall size. Otherwise, two distinct architectures must be designed, one for the wall-mounted configuration and the other for the ceiling-mounted configuration, which also represents a penalty.
The documents WO 95/23441, EP-A-0 805 508, EP-A-0 521 326, xe2x80x9cAnalysis and design of a circumferential wide slot cut on a thin cylinder for mobile base station antennasxe2x80x9d (J. Hirokawa et al., IEEE, Proceedings of APSIS, 1993, Vol.3, Jun. 28, 1993, pages 1842-1845), and the abstract of Japanese Patent Application JP-A-09 232835, disclose antennas whose radiating elements are radiating slots.
The document GB-A-2 229 319 discloses an antenna intended to be mounted in a vertical configuration and in which the radiating element is a pair of spaced parallel metal plates. It is stated that the antenna could be mounted horizontally, in a ceiling or in a floor.
An object of the present invention is to propose base stations whose antenna systems are well suited to the usual indoor installation conditions and do not require duplication.
The invention therefore proposes a radiocommunication base station including, for communicating by radio with mobile stations, at least one antenna system including a casing for fixing it to a support.
According to a first aspect of the invention, the antenna system includes at least one radiating slot formed in a conductive plane parallel to a front face of the casing and adapted to emit, in a first direction substantially perpendicular to the front face of the casing, an electric field whose polarisation is oriented in a second direction substantially parallel to said front face and perpendicular to the orientation of the slot, and to emit, in at least one other direction, substantially closer to the second direction than to the first direction, an electric field whose polarisation is oriented substantially in the first direction, the casing having a first operating position in which said second direction is substantially vertical and a second operating position in which said first direction is substantially vertical.
In the wall-mounted configuration, the antenna system is disposed so that the xe2x80x9cfirst directionxe2x80x9d is vertical. Mobile stations facing the antenna system thus receive a wave whose electric field has a relatively strong vertical component, which is what is required.
In the ceiling-mounted configuration, the front face of the casing is horizontal. Because the xe2x80x9cother directionxe2x80x9d is oriented towards an area to be covered, mobile stations in that area also receive a wave whose electric field has a relatively strong vertical component. It is true that the directly radiated electric field is quasi-horizontal at positions vertically aligned with the casing. However, because the mobile stations that are located there receive a relatively high power, this orientation of the electric field does not give rise to any sensitivity problem. To the contrary, radiating a quasi-horizontal electric field in the immediate vicinity of the station means that depolarisation losses limit the incidence of blocking problems, i.e. problems of saturation of receivers (see GSM Specification 05.05). These blocking problems are very serious in practice and currently give rise to very tight specifications for the linearity of receivers, which is an additional-cost factor.
According to a second aspect of the invention, the antenna system includes two parallel radiating slots oriented perpendicularly to the first and second directions and separated by a distance substantially equal to half the radiated wavelength and means for feeding radio-frequency energy to the two slots adapted to energise the two slots either in phase or in phase opposition according to whether the front face of the casing is installed in a horizontal or vertical plane.
Other features and advantages of the present invention will become apparent in the course of the following description of non-limiting embodiments of the invention, which description is given with reference to the accompanying drawings, in which: